A chalcogenide-based material, for example, has the property of being stable in either a crystalline phase or an amorphous phase at room temperature, and its resistivity changes by two to four orders of magnitude between the two phases. A nonvolatile memory is realized by making use of this property. In other words, information is written by setting a thin film of such a material, which is stable in either the crystalline or the amorphous phase, and the information is read by detecting, through the measurement of its resistance value, whether the thin film is in the crystalline phase or the amorphous phase.
When writing information, i.e., an 1 or an 0, to such a memory, the thin film of the phase-change material must be made to undergo a phase change from crystalline to amorphous or from amorphous to crystalline. Generally, a chalcogenide-based material solidifies into an amorphous phase when the material is heated to 630° C. or higher and then quickly cooled. On the other hand, when the material is heated to 200° C. or higher and then gradually cooled, it stabilizes in a crystalline phase. The thin film of the phase-change material is heated by using the Joule heat generated when a current flows through the thin film. When the thin film of the phase-change material has changed to an amorphous phase, the resistance value of the thin film is two to four orders of magnitude larger than that when it has changed to the crystalline phase. Accordingly, by applying a read voltage to the thin film of the phase-change material and detecting the amount of current that flows, it can be determined in which phase, the amorphous or the crystalline phase, the thin film remains stable, thus enabling written information to be read out.
Recently, it has been found that, in this kind of phase-change thin film, the amount of current can be controlled by applying a bias voltage perpendicular to the flow direction of the current, and by doing so, a phase-change channel transistor having a memory function as well as a switching function has been proposed (Japanese Unexamined Patent Publication No. 2005-93619). In this phase-change channel transistor, the memory function is achieved by forming the channel portion from a phase-change material, and the information read/write timing can be controlled by switching the current flowing through the channel portion on and off by gate voltage. When RAM is constructed using such phase-change channel transistors, each select transistor and its associated memory part can be implemented in a single transistor, and an ultra high-density storage device can be achieved. In a traditional DRAM, on the other hand, each memory cell comprises a select transistor and a memory element formed from a capacitor, and the area of the memory cell increases because of the need to fabricate the capacitor on a semiconductor substrate, which has been a factor impeding device miniaturization. Therefore there has been a limit to the extent to which memory cell density can be increased.
In a phase-change channel transistor, as well as in a phase-change memory, when writing information the phase-change material layer must be made to undergo a phase change. To effect this phase change, Joule heat must be generated by flowing a current through the thin film of the phase-change material. However, for a phase change from an amorphous to crystalline phase, considerably high voltage must be applied in order to heat the material to a temperature required to cause a phase change, because in the amorphous phase, electrical resistance is high and current does not easily flow. As a result, write voltage to the memory device increases. Furthermore, when the material has changed from an amorphous to crystalline phase, since the resistance is low in the crystalline phase, excessive current flows through the material because of high voltage, and the device may break down.
To prevent this, there is a need to reduce the amount of resistance in the amorphous state of the phase-change material, thereby reducing the voltage to be applied for a phase change from the amorphous to the crystalline phase. It is desirable that the change in resistance between the amorphous and crystalline phases, which, with the present state of the art, is as large as two to four orders of magnitude, be reduced to one order of magnitude or less. However, phase-change materials that can satisfy such requirements have not been developed yet.